The present invention relates to an acquisition device for digitizing highly dynamic signals after possible amplification.
Highly dynamic analog signals are obtained in numerous fields and particularly in the field, of seismic prospecting.
1. Field of the Invention
Methods of seismic prospecting generally comprise the transmission of waves by a seismic source, the reception of these waves after they have been reflected by the different reflecting interfaces of the sub-soil, and the production of seismic recordings representative of the configuration of the sub-soil. The reception of the reflected waves is done by reception devices including a very large number of sensors spread out along a seismic profile to be studied.
For seismic off-shore prospecting, the sensors are spread along and inside a sealed flexible sheath of great length called a seismic streamer.
The assembly of sensors is generally split into a plurality of subassemblies. The signals picked up by the sensors of the same sub-assembly are very often collected by an electronic data acquisition apparatus which digitizes them and records them. At given intervals, the signals recorded by the different acquisition apparati are transmitted sequentially to a central control and recording laboratory through a multiplexed transmission system by means of transmission lines, or by radio. Such a transmission system is described, for example, in French patent No. 2,417,088.
To obtain greater and greater resolution power, seismic reception devices have been developed having an increased number of seismic sensors, which increases accordingly the number and complexity of the acquisition apparati used for collecting all the received signals. It is therefore desirable to employ acquisition simultaneously for performance reliability and relatively low electrical consumption.
The acquisition devices used especially in seismic transmission-reception systems comprise, for example, an analog multiplexer whose different inputs are connected to the different signal sensors, either directly or through fixed gain pre-amplifiers and filters. The output of the multiplexer is connected to a single amplifier formed of several stages connected together in series or in parallel. Each of the stages may be associated with a feed-back network. A selection member chooses the number of amplifiers and/or their respective gains so as to give to each amplified signal a suitable amplitude before applying it to an analog to digital (A-D) converter. The latter transforms each signal sample into a digital word by comparing its amplitude with a fixed reference signal which is applied thereto.
Different amplifiers are described for example in the French patent No. 2,234,709 or the U.S. Pat. Nos. 4,031,504, 3,754,240. The application to a signal of a gain or optimum attenuation very often requires the provision of switching so as to vary the number of stages through which the signal passes and/or the gain or the attenuation applied of each stage and there then arise tuning difficulties due to so-called "offset" voltages, non linearity of the stage and saturation risks. These difficulties are all the greater when amplification stages with limited performance are used for reasons of economy of price or electric consumption, particularly.